In order to meet the ANSI standard T1.601 for 2B1Q modulation, two-wire, full-duplex data transfer with echo cancellation, currently installed ISDN basic rate digital subscriber lines (having a data rate of 144 kb, with bidirectional data payload, plus overhead maintenance channels) must not exceed a two-wire loop loss of 42 dB at 40 KHz, or 1300 ohms, resistive. These requirements effectively limit the operational range of such a two-wire loop to approximately 15.2 kft, using No. 26 (American Wire Gauge) wire, and commercially available ISDN trasceiver interface equipment. Extending ISDN communications to customers geographically located beyond this range requires the service provider to either install repeaters in the loop, or to use a different communication medium, such as a T1 carrier fiber optic link. Unfortunately, each of these alternative solutions to the extended range problem has a penalty attached to it.
The ISDN repeater approach requires both an office end repeater powering unit, and a repeater mounting pole, or a subterranean (bell jar) environmentally hardened housing for the repeater. This not only involves the use of additional equipment (including the cost of the hardware and its installation), but the expense and labor of maintaining the repeater enclosure. Similarly, although T1 channel banks, located in both the ISDN switch office and another downstream location (office or remote hut) that is geographically `close` to the subscriber premises, are able to accept basic rate interface transmission extender (BRITE) cards for T1 carrier extension, the fact that T1 carrier systems are configured to include capacity for multiple extended basic rate services means that their use to deliver only a single basic rate extended service is prohibitively expensive and impractical.